1. Field of the Invention
This invention concerns the treatment of waste anesthetic gases produced by one or more anesthesia delivery systems of a healthcare or other facility that use inhaled anesthetics for medical, dental, or veterinary purposes. In order to prevent atmospheric pollution, the invention pertains to the removal and reclamation of nitrous oxide, fluoro-ethers, and other halocarbons from a stream of waste anesthetic gases prior to its discharge to the atmosphere. In particular, this invention involves the removal and reclamation of anesthetic gases at elevated pressures, which allows the removal and reclamation process by condensation to be conducted at higher temperatures.
2. Backround Art
Anesthesia delivery systems in surgical facilities (medical, dental, and veterinary) produce significant quantities of waste anesthetic gases. Currently these gases are collected from the patients' exhalation by a dedicated or shared vacuum system. The healthcare facilities typically employ one or more centrally-located vacuum pumps to collect waste gases from individual anesthetizing locations. These vacuum pumps are usually oversized, because they are designed to collect exhaled anesthetics over a wide range of flow rates. Because these pumps operate continuously, the waste anesthetic gas suction system also entrains large amounts of surrounding room air from the anesthetizing locations, significantly diluting the waste anesthetic gases therein. At the central vacuum pump(s), the gas stream is often admixed with additional room air to further dilute it prior to its ejection from the facility. This dilute waste anesthetic gas/air mixture is typically pumped to a location outside of the surgical facility, where it is vented to the open atmosphere.
The waste anesthetic gases are generally collected at about 20-30° C. with a relative humidity ranging between 10 to 60 percent. The average composition of the waste gases is estimated to be (in volume percent): 25-32 percent oxygen, 60-65 percent nitrogen, 5-10 percent nitrous oxide, and 0.1-0.5 percent volatile halocarbons, including fluoro-ethers, such as isoflurane, desflurane and sevoflurane. The waste anesthetic gas may also contain trace amounts of lubricating oil vapor from the vacuum pumps.
An increasingly significant source of environmental concern, waste anesthetic gas halocarbons (similar in composition to Freon-12® and other refrigerants) have been linked to ozone depletion and to a lesser degree, global warming. The halocarbons used in anesthesia (primarily halogenated methyl ethyl ethers) now represent a significant emissions source, because other industrial and commercial halocarbon emissions have been greatly reduced by legislation and other initiatives in recent years. Although waste anesthetic gas emissions have so far escaped environmental regulation in the United States, legislative initiatives to strictly regulate waste anesthetic gas emissions will likely occur in the near future.
Several techniques have been proposed to treat waste anesthetic gases in an attempt to mitigate the growing problem of waste anesthetic gas emissions. For example, U.S. Pat. No. 4,259,303 describes the treatment of laughing gas with a catalyst, U.S. Pat. No. 5,044,363 describes the adsorption of anesthetic gases by charcoal granules, U.S. Pat. No. 5,759,504 details the destruction of anesthetic gases by heating in the presence of a catalyst, U.S. Pat. No. 5,928,411 discloses absorption of anesthetic gases by a molecular sieve, and U.S. Pat. No. 6,134,914 describes the separation of xenon from exhaled anesthetic gas. A cryogenic method for scrubbing volatile halocarbons from waste anesthetic gas is disclosed by Berry in U.S. Pat. No. 6,729,329, which is incorporated herein by reference.
Another cryogenic waste anesthetic gas condensation system has recently been disclosed by Berry, et al. in co-pending application Ser. No. 11/432,189, entitled “Anesthetic Gas Reclamation System and Method.” This system uses a batch-mode frost fractionation process whereby the temperatures of the individual anesthetic gases are lowered to a point such that they condense and collect as frost on the cooling surfaces of a cold trap/fractionator. This co-pending application, filed on May 11, 2006, is incorporated herein by reference.
FIG. 1 illustrates a typical waste anesthetic gas reclamation system 10 of prior art for a healthcare facility. The system 10 includes a number of individual anesthetizing stations 15A, 15B, 15C, each having an anesthetizing machine 12A, 12B, 12C which delivers anesthesia to a patient via a mask 14A, 14B, 14C or similar device. Excess anesthetic gases, patients' exhalation, and air are collected at the masks 14A, 14B, 14C by the anesthetizing machines 12A, 12B, 12C and discharged to a common collection manifold 16. The waste anesthetic gas collection manifold 16 is typically hard plumbed into the healthcare facility, and the anesthetizing machines 12A, 12B, 12C are removably connected to the collection manifold 16 at standard waste anesthetic gas connectors 18A, 18B, 18C, e.g. 19 mm or 30 mm anesthetic connectors. The waste anesthetic gas reclamation system 10 operates at a vacuum pressure which is generated by one or more central vacuum pumps 20. The collected waste gas stream is typically passed through a check valve 35 to a condenser unit 22 consisting of one or more heat exchangers. A source of liquid oxygen, or other suitable heat sink, extracts heat from the waste anesthetic stream, condensing the anesthetic gas components. The liquid anesthetic condensate is captured in collection vessel 24, and any liquid water condensate is captured in collection vessel 23. The remaining gas stream, stripped of waste anesthetic gas components, passes through a receiver 26 and the vacuum pump(s) 20, and it is then exhausted to the atmosphere outside of the healthcare facility through vent 46.
The current methods for scavenging waste anesthetic gases from anesthetizing locations 15A, 15B, 15C in healthcare facilities generally involve drawing high flows of room air into the dedicated or shared vacuum collection manifold 16 to entrain waste anesthetic gases. The collection manifold 16 may also continuously draw in air through a number of idle anesthetizing machines 12A, 12B, 12C. On average, the collection system manifold 16 extracts between 20-30 liters of waste anesthetic gas and/or room air per minute at each anesthetizing location 15A, 15B, 15C. For a large hospital having between 20-30 operating rooms, it is estimated that waste anesthetic reclamation system 10 flow rate ranges between 500-1000 l/min. (14-35 scf/min.).
The advantages of a high-flow dilute waste gas system are that the system easily accommodates a wide range of anesthetic exhaust flows, the system is safer because little anesthetic can escape the system, and the system is more trouble-free because little maintenance is required. However, high-flow systems are energy-intensive, generally requiring large vacuum pumps 20 in order to maintain sufficient suction at a large number of anesthetizing stations 15A, 15B, 15C. For example, in order to maintain a vacuum of about 200 mm Hg at a flow rate of 1-2cubic feet per minute (cfm) at each anesthetizing station 15A, 15B, 15C, vacuum pumps of 100-200 cfm capacity are not uncommon.
Additionally, a diluted waste anesthetic gas stream is thermally inefficient. Removal of a waste gas component by condensation requires lowering the temperature of the entire flow stream to a point where the partial pressure of the gaseous waste component is equal to or greater than its saturated vapor pressure (at that temperature). Therefore, to cool the large volume of diluted waste anesthetic gas to a temperature below the saturated vapor pressure of its components, a sizeable cooling utility (i.e. a greater quantity of liquid oxygen, liquid nitrogen, etc.) is required. A method and system for increasing the efficacy and efficiency of condensation-type waste anesthetic gas scavenging and reclamation systems are thus desirable.
A low-flow scavenging system provides a more efficient means of waste anesthetic gas recovery through condensation, because a smaller volume of gas has to be cooled to the condensation temperatures of the individual gases. A low flow scavenging method, facilitated by a dynamic waste anesthetic gas collection apparatus, has recently been disclosed by Berry et al. in co-pending application Ser. No. 11/266,966, entitled “Method of Low Flow Anesthetic Gas Scavenging and Dynamic Collection Apparatus Therefor.” This co-pending application, filed on Nov. 4, 2005, is incorporated herein by reference.
Typically, anesthetic gases are highly volatile substances. For a given temperature, they have a higher vapor pressure than the vapor pressure of water and other lower volatile substances. Substances with higher vapor pressures generally require greater cooling to achieve the same or similar condensate recovery as substances with lower vapor pressures. Thus, anesthetic gases need to be cooled to extremely low temperatures, i.e. cryogenic temperatures, in order to recover appreciable amounts of anesthetic as condensate. However, these extremely low temperatures approach, and in many cases, fall below the freeze point of many anesthetics. In such situations, the waste anesthetic gas stream may still contain anesthetic concentrations that could be condensed except for the undesirable freezing of the system.
Pressure, in addition to temperature, can greatly influence condensation. Elevating the pressure of the condensation system is advantageous, because it allows condensation to occur at significantly higher temperatures than would otherwise occur at lower operating pressures. This also avoids the risk and problems associated with freezing of the condensate. For these types of vapor/liquid phase equilibrium systems, the most beneficial thermodynamic characteristic is that pressure has a much larger effect on the dew point of the vapor than the freezing point of the liquid. Thus, the dew point temperature of a typical anesthetic-laden vapor stream increases with increasing pressure while its freezing point temperature stays relatively constant for varying system pressures.
The increased temperature span between the dew point of the vapor and the freeze point of the condensate, due to increases in system pressure, provides greater operational flexibilities for condensation systems. For example, less cryogenic refrigerant is needed to effect the same amount of condensation, because condensation can occur at higher temperatures. Furthermore, if a more complete separation of the anesthetic from the waste gas stream is desired, the system temperature can be lowered while maintaining an elevated pressure. This permits additional anesthetic to be condensed from the vapor phase without the associated risk of condensate freezing. Thus, a strategy may be developed to achieve the optimum separation of anesthetic by simply adjusting the condensation system pressure relative to the condensation system temperature. Of course, the relative refrigeration versus compression costs should also be considered in any cost optimization strategy.
3. Identification of Objects of the Invention
A primary object of the invention is to provide an economical system and method for removing fluoro-ethers, nitrous oxide, and other volatile halocarbons from waste anesthetic gases from a surgical or other healthcare facility before such gases are vented to the atmosphere.
Another object of the invention is to provide an economical system and method for substantially preventing atmospheric venting of fluoro-ethers and other volatile halocarbons of waste anesthetic gas while eliminating the need of prior art catalysts, charcoal granules and heating techniques.
Another object of the invention is to provide a system and method which reclaims and allows re-distillation and/or reuse of a large percentage of the nitrous oxide and/or anesthetic halocarbons used in the facility.
Another object of the invention is to provide an economical system and method which utilizes and enhances existing waste anesthetic gas reclamation systems of healthcare facilities for minimal impact and cost.
Another object of the invention is to provide an economical system and method which utilizes existing liquid oxygen and/or liquid nitrogen storage and delivery systems of healthcare facilities for energy efficiency and minimal impact during the reclamation system installation.
Another object of the invention is to provide an economical system and method for separating various removed nitrous oxide, fluoro-ethers, and other volatile halocarbon components based on their characteristic bubble and dew points.
Another object of the invention is to provide an economical system and method for increasing the efficacy and efficiency of condensation-type waste anesthetic scavenging systems.
Another object of the invention is to provide a flexible system and method for increasing the efficacy and efficiency of condensation-type waste anesthetic scavenging system by operating the system under varying pressures and temperatures.
Other objects, features, and advantages of the invention will be apparent to one skilled in the art from the following specification and drawings.